1. Field of the Invention
The present invention relates to a method for preparing diamond coated substrates, and resultant diamond coated substrates. More specifically, the present invention is directed to a sintered body having a composition comprising a silicide-containing material selected from the group consisting of silicon carbide, silicon nitride, and silicon nitride composite materials coated with a diamond film. Preferably, the present invention is directed to a diamond coated sintered body having a composition comprising microcomposite materials, such as cobalt silicide-containing microcomposite materials, and most preferably cobalt silicide-containing silicon nitride microcomposite materials, which may be produced by coating the microcomposite material, such as with a polycrystalline diamond film. The diamond film on the microcomposite sintered bodies produced in accordance with the present invention is hard and abrasion-resistant. Thus, the diamond coated sintered bodies comprising cobalt silicide-containing microcomposite materials, such as cobalt silicide-containing silicon nitride microcomposite materials, are particularly useful as cutting tools for turning, drilling and milling of nonferrous alloys, composites, wood, plastics and other materials.
2. Discussion of Background Information
The mechanical, thermal, electrical and optical properties of diamond films offer major advantages in a number of areas ranging from hard surfaces to semiconductors. For example, a diamond-coated carving knife remains sharper, and heads of computer hard disk drives, which occasionally come too close to the surface of the disk, with catastrophic results, may be protected by a diamond coating which is only about 0.03 .mu.m thick.
The high thermal conductivity of diamond which, at room temperature, exceeds that of any other material, and electrical insulating properties of diamond make diamond coatings especially attractive to semiconductor technology. Diamond is unique in that no other material is simultaneously such a good heat conductor and a good electrical insulator. Diamond coatings thus offer the possibility of more densely packed microchips i.e., more transistors per chip with an attendant increase in computer power.
Diamond coatings also offer the possibility of replacing silicon semiconductors, inasmuch as they can be doped to form p- and n-type materials. Inasmuch as diamond has a large electron band gap compared to silicon, the larger electron band gap makes diamond less sensitive to thermal or radiation damage than silicon. Thus, diamond transistors may be used in hostile environments.
Another characteristic of diamond which makes it attractive as a semiconductor is that the high velocity of electron transmission through diamond increases smoothly with increasing electric fields. This is unlike many types of semiconductors. For example, the electron velocity in diamond can be even greater than in gallium arsenide, which is currently considered to be the most expedient high speed semiconductors.
This high electron velocity, coupled with the ability of diamond to sustain a greater electric field than other semiconductors before breakdown, makes diamond a particularly suitable material for high power telecommunication systems.
The optical properties of diamond also make it suitable in a variety of applications. Pure diamond is transparent to photons from the far IR (80 .mu.m) to the UV (230 nm), so diamond coatings have potential for many window applications.
The preparation of diamond coatings using plasma chemical vapor deposition (CVD) has been reported in the prior art. Plasma CVD involves forming a plasma containing activated species using a direct current (dc), low frequency, radio frequency (rf) or microwave discharge, and then contacting a substrate with the plasma to form a diamond coating.
Electrical discharges, such as glows, coronas, arcs, radio frequency and microwave have been studied for many years as tools for forming plasmas containing activated species. Such discharges can generally be divided into low temperature and high temperature systems. Low temperature systems include glows, coronas, electrodeless discharges, and the so-called ozonator type discharges, as well as some RF and microwave discharges, and are collectively known as silent discharge systems. Chemical activation in low temperature systems results in a non-equilibrium product of activated species such as ions and free radicals. The concentration of active species in such plasmas is much greater than would be expected on the basis of equilibrium, i.e., thermal considerations.
In contrast, in high temperature discharges, such as an electrical arc, the concentration of activated species depends upon attaining a thermodynamic equilibrium favoring the activated product. This means that the process of producing active species is inefficient, most of the heat being used to heat the gas. In order to produce a significant amount of activated species, extremely high temperatures, e.g., temperatures of at least 4000.degree. K. are required. Temperatures as high as 10,000.degree. K. to 30,000.degree. K. are not uncommon in arcs. However, the use of such high temperatures in preparing diamond films presents difficulties. Not only does it require a great amount of power, but it results in heating the substrate to the extent that graphite and amorphous carbon may be formed instead of diamond and even in the destruction of the substrate. It also limits the types of substrates that can be used due to problems with the thermal stability of the substrate and differences between the coefficients of expansion of the substrate and the deposited diamond film.
The preparation of diamond films by high temperature plasma CVD is described in Kurihara et al., Appl Phys. Lett., 52(6) 19: pp 437 and 438 (1988) and in European Patent Application 0286306, filed Mar. 30, 1988. In one embodiment a plasma is formed by passing a mixture of methane and hydrogen through an arc discharge. The arc discharge produces activated species in a thermal plasma. The plasma is then passed through a nozzle, expanded to precipitate diamond and then directed onto a substrate.
Methods for preparing diamond films using non-equilibrium plasma CVD have been described.
U.S. Pat. No. 4,767,608, MATSUMOTO et al. is directed to the preparation of carbon films by high temperature plasma CVD, wherein the formation of diamond involves the use of an electric arc discharge which utilizes high temperatures to produce significant amounts of activated species.
The use of a non-equilibrium plasma jet is described in a report entitled "Synthesis of Silane and Silicon in a Non-equilibrium Plasma Jet," authored by H. F. Calcote (Department of Energy/Jet Propulsion Laboratory Report 954560-76/8) which discloses the preparation of amorphous and polycrystalline silicon films which are strongly adhered to Pyrex, Vycor, aluminum or carbon. The plasma is formed using a DC discharge and subsequently expanded through a nozzle to form a plasma jet which is directed upon a substrate. This non-equilibrium plasma jet provides the source of hydrogen atoms via the dissociation of hydrogen gas in an electrical discharge to react with silicon halides.
Diamond coatings have been applied to a carbon steel substrates by first depositing a thin (.about.200 angstrom) coating of silicon on the substrate. (Appl. Phys. Lett. 58(4), 358-360 (1991). Subsequent diamond coating was carried out using a microwave plasma reactor.
U.S. Pat. No. 4,990,403, entitled "Diamond Coated Sintered Body," describes the significance of being able to deposit a diamond coating on a tough substrate material, such as tungsten carbide. This patent discloses a diamond coated sintered body consisting of a tungsten carbide that contains carbide and/or nitride phases. The carbide and/or nitride phases present in the substrate promotes adhesion of the diamond to the sintered body. Sintering aids such as yttria, alumina, zirconia and magnesia are substituted for cobalt sintering aids which are conventionally used for tungsten carbide.